Radio compass



June 10, 1947. E D, BLODGETT 2,422,072

I RADIO COMPASS I Filed Feb. 1'5, 1945 ra Pan/2'@ Y suppl: Y l

:inventor Bf LAL Gttorneg Patented `lune 10, 1947 EdwardD. Blodgett,Haddoneld. N. J., assigner to Radio Corporation of America, acorporation of'Delaware Application February 15, 1943?, SerialNo;475,858-

(Cl. Z50- 11) Y 8 Claims.

l This invention relates toradio direction finding and more particularlytoy the conversiont of a radio direction finder toprovide-visual'left-right course indications, for homing, ortravelingalong a predetermined course.

An. aurall null' typeA of direction finder fitted with meansl fordetermining bi-lateraland unilateral bearings is a common installationon aircraft. Such aircraft direction finders comprise a rotatable loop,a radio receiverA and` an output indicating devicewhich mayy bea. meteror headphones. To determine the bearing of a radio transmitter withsuchequipment the loopiis rotatedftoa position such that minimum outputis provided by the receiver. The axisofV the loop iszthen directed alongthe line betweenthetrans mitter and; the receiver and the azimuth angleis indicated by an index mechanically connected to the loop'. Thedirective pattern of the loop antenna is a igurefof-eight whichprovidestwo minima.` in the receiver output as the loop is7 rotated. To resolvethe ambiguityv of. the reciprocal bearing-` produced by this effect, anondirective antenna is usuallyy provided; and; the outputs of theloopand nonfdirective antennas arecombined to provide a: cardiod.directive pattern; The single minimum` of thisl pattern is directedalong: the plane ofthe loop invone-direction or the-other from thecenter dependingupon thev polarity in: which t-he loop isv connected tothe-receiver.

It; is the principal object of the-present invention to provide animproved means for convert ingV a direction finder of the abovedescribedt-ype so as to produce visual` left-right course, indications.

A-nother object is to'provide-an improved means for' convertingexisting. direction finders into leitright indicators withoutthenecessityv of altering ther original installation.

A further object is to providel an improved methodw of iand meansforsuch conversion, involvin'g the addition of a minimum quantityoiapparatus: These and other and'incidentalobjects will become apparentto those skilledin the art upon consideration of the' followingdescription, with. reference tothe accompanying drawing, of which, Eig;l is aschematic. diagram o a direction'. nder embodying the:inventiongrFigi. 2 is a graph of the cycle oi operation of one of theswitches inthe system of, Fig.. 1, Eig. Slis. a graphicalrepresentation4 of; the rectified output ofy the radio. receiver of.Fig. 1j andFig. 4 is* a graphical representMion. of' the voltages'appledto-the indlcator'of'thezsystem of Fig. 1'.v

Referring to- Fig; 1;, a loop antenna l; is; connectedV through a;reversing switch 3- to af radio receiver. 5. fr non-directive antenna.l: is also connected to the receiver' 5; The receiver 5 is of theconventional super-hete-rofdyne type and, together with the antennas I'-and 7:, may be part of! a previously.' installed direction findensystem.

A heatoscillator-Q.- is coupled tothe-L-Rampli'- er of the receiver 5.1The output circuito the receiver 5; is connected. to ay rectier Iii. Theoutput circuitl of the rectifier Hfis. applied through asinglepole-double throwfswitclr- I3.. to^ a meter l5. A pair ofcapacitors IS: and= 2i arel con.- nectedfacross the meter I5;andthe-return circuit of the rectier H is connected'to-their commonjunction. A capacitor 22- is connected' across the meter to smooth outfluctuationszin thefvoltf age across thel meterl and to provide-vdamping. rEhe switches 3 andi I3 are arrangedy to begopefrated invsynchronism; by4 amotor H, which? is connected through a: switch: Ittothe,V receiver power: source; not shown. A switch. is ganged with: theswitch.y i4' and arranged to connect the loop ldirectly to` the receiverinput whenI the motor l'l is deenergized. Thus-theloopf-is alwaysconnected in the same polarity when: the motor switch, is:o1f.v

Referringl to Fig, 2; the yc-leofopenation of the Switchs 3 is such'that during thefperiodsPl theloop is connected-tothe receiver inapolarity arbitrarily designated' as positive.. Dur-ing the periods P2-the loop is. connected toi thez receiver in the-opposite', or negati-vepolarity. The rate of oper-ationvl of the switches- 3= and I-isapproxi:- mate-ly 10 cycles per second. Since this. ireduency is. wellbelow the low frequency cut off of the output or audio channel-ofthestandard aircraft receivenit is necessary to provide other modulation ata higher frequency. Ther beat oscillator Slis tuned toafrequencydiieringV from that of' the I.f-F. frequency` of the receiver 5 by, forexample, 1000` cycles,- orfrequency corresponding approximately. to4 themid-frequency of the audio or output response characteristic ofReferring to- Fig. 3, assuming that thev loop- I is. olf course to. theleft, the voltage from the rectier l-!- during the-period-PI-wi-ll be ofgreater amplitude than that produced during the period P2. In each case,transients are induced at the beginning of the period by the operationof the switch 3, as indicated at A. The switch I3 connects the output of the rectifier II to the capacitor I9 during the periods PI, and to thecapacitor 2| during the periods P2. The mechanical connection betweenthe switches 3. and I3 is such that the switch I3 closes at a time Tafter the closure of the switch 3`. 'I'hus the voltage applied to thecapacitor I9 is of the form shown at L in Fig. 4, and the voltageapplied to the capacitor 2l is of the form shown at R in Fig. 4. 'I'hecapacitors I9 and 2| are charged to corresponding voltages. Thesevoltages are applied in opposition to each other to the meter I5.

Y When the condensers I 9 and 2l are charged to equal voltages, nocurrent flows through the meter. When the two voltages are unequal, asunder the conditions illustrated in Fig. 4, an equalizing current owsthrough the meter I5, causing a deflection to the left. Similarly whenthe axis of the loop is to the right of the course line, the meter I5 isdeflected to the right.

If copper oxide contact rectifier elements are used for the rectifier lI of Fig. 1, it is found that operation is unsatisfactory at lowtemperatures. This is remedied by enclosing the rectifier in a heatinsulated box 8 with a heating element I0 and a thermostaticallycontrolled switch I2. The heater is connected to the motor I1 so thatthe temperature of the rectifier is maintained at the proper levelduring operation of the system.

Since the antenna switching is done in the loop circuit, rather than inthe circuit of the nondirective antenna, modulation of Vthe carrier,when the loop is on course, is avoided. The transient caused Iby theclosure of the switch 3 is of the same duration regardless of theswitching rate. Hence the switching rate is madevery slow, so that thetransient period is a small fraction of each switching cycle. This hasthe additional advantage of producing a minimum audio interference whenthe operator is listening to or monitoring the received signal Prior artradio Compasses of the left-right indicator type require that the audiofrequency system of the receiver be designed to pass the loopmodulation, or switching frequency. If all of the advantages of lowfrequency switching were to be retained, it would be necessary toprovide unusually large transformers or other coupling devices in theaudio channel. This is avoided in the present invention by the use ofthe beat frequency oscillator, which beats with vthe incoming signalfrequency to provide currents of the required frequency.

Thus the invention has been described as an improved method of and meansfor converting a conventional direction finder of the type used on boardaircraft to provide visual course indication. The originally installedequipment is left unaltered and a conversion unit comprising motordriven switches, a rectifier, and an indicator is connected thereto. Theswitching operation is performed in the loop or directional, rather thanthe nondirectional circuit, to eliminate modulation of the receivedcarier when on-course, i. e., with the loop in the null position. 'I'heswitching operation is performed at a low frequency rate which in eiectreduces the total amount of contact noise over a given time interval.Since the contact noise is of fixed duration, regardless of switchingspeed, the low frequency switch rate provides the maximum usable timenterllil duif* 4 ing which output voltage may be applied to theindicator.

I claim as my invention:

l. A radio direction finder converter of the leftright indicator typecomprising a loop antenna, a radio receiver including audio ampliermeans having a response characteristic extending over a predeterminedfrequency range, with a predetermined low cutoff frequency, a reversingswitch connected to said loop and to the input circuit of said radioreceiver, a beat frequency oscillator connected to said receiver, arectifier connected to the output circuit of said receiver, a secondreversing switch connected to the output circuit .said receiver by afrequency approximately equal to the mid-frequency of said audioresponse characteristic of said receiver.

4. The invention as claimed in claim l wherein said receiver is of thesuperheterodyne type, having a predetermined I.F. frequency, and saidreversing switches are operated at a frequency of approximately 10cycles per second and. said beat oscillator operates at a frequencydiffering from said I.-F. frequency of said receiver by approximatelycycles per second.

5. The invention as claimed in claim 1 wherein the circuit of saidindicator includes a pair of capacitors connected to said secondreversing switch so as to be charged separately and alternately, and aD.C. meter connected to said capacitors so as to provide a path for theow of current to equalize the charges on said capacitors.

6. The invention as set forth in claim l wherein said receiver is of thesuperheterodyne type, having a predetermined I.-F. frequency, and saidreversing switches are arranged to operate at a frequency below thelower limit of said audio response characteristic of said receiver, andsaid beat frequency oscillator is arranged to operate at a frequencydiffering from sai-d I.F. frequency of said receiver by a frequencywithin the audio response band of said receiver.

7. A left-right indicator convertor for a radio direction finder whichincludes a directive antenna and a radio receiver including audioamplifier means having a predetermined low cutoff frequency, saidconvertorA comprising a reversing switch connected to said directiveantenna and connected to the input circuit of said radio receiver, abeat frequency oscillator connected to said receiver, a rectifierconnected to the output circuit of said receiver, a second reversingswitch connected to the output circuit of said rectifier, indicatormeans connected to said second switch, and a motor arranged to operatesaid switches in synchronism at a frequency lower than said cut offfrequency.

8. The invention as set forth in claim 7, including an auxiliary switchconnected to said directive antenna and connected to the input circuitof said receiver, a motor control switch in the power input circuit ofsaid motor, and a mechanical con- 5 6 nection between said tWo I astmentioned switohes UNITED STATES PATENTS so arranged tha-t When saidmotor control sWltch is opened, said antenna, is connected through saidNumber Name Date auxiliary switch to said receiver. 2,256,619 Luck Sept.23, 1941 5 2,275,296 Hagen Mar. 3, 1942 EDWARD D. BLODGE'IT 2,218,361Runge ei', 2,1 Oct. 15, 1940 REFERENCES CITED FOREIGN PATENTS i NumberCountry Date The following references are of record in the 10 460,271Germany May 21, 1923 file of this patent; 542,427 Germany J an, 23, 1932

